Thermionic device



y 1967 R. A. CHAPMAN THERMIONIC DEVICE 3 Sheets-Sheet 1 Filed July 29, 1963 HEAT IN HEAT OUT 6 Fig. I

VACUUM LEVEL T TER AT TEMP TE Fig. 2

COLLECTOR AT TEMP T FERMI LEVEL Richard A. Chapman mvzm-oa ATTORNEY July 25, 1967 THERMIONIC mavrcn R. A CHAPMAN 3,333,140

5 Sheets-Sheet 2 Filed July 29, 1963 COMPOSITE JE COLLECTGR EMITTER AT TEMP T c vAauuM LEV/Ell. TAT TEMP TE iox i METAL INSULATING z lgf f 'y SUBSTRATE LAYER I E Fig. 3 V

COMPOSITE COLLECTOR AT TEMP T J C Ev T it? 'EITE T PLASM VEL AT T Ag SCH E C c ox I x' METAL FERMI LEVEL/ V i METAL "NSULAT'NG EMITTER FERMI -I T E T C SUBSTRAT LAYER E |;v E| 7 F lg. 4

Richard A. Chapman INVENTOR ATTORN EY July 25, 1967 R. A CHAPMAN 3,333,140

THERM IONIC DEVI GE Filed July 29, 1965 s Sheets-Sheet s Fig. 5 C' 8' A |o' l/ F E 5 IO E D o Z 0 E -9 0 l0 w .1

OUTPUT VOLTAGE (VOLTS)----- Richard A. Chapman INVENTOR BYW Flg. 6

AT TO RN EY United States Patent 3,333,140 THERMIONIC DEVICE Richard A. Chapman, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed July 29, 1963, Ser. No. 298,065

7 Claims. (Cl. 313-346) The present invention relates to a thermionic device for converting heat energy to electrical energy. More specifically, it relates to a thermionic converter having an improved composite material electron collector.

The maximum output power which can be derived from a thermionic converter is equal to the saturated emitter electron current from a hot, high work function emitter electrode times the difference in the work function of the hot, high work function emitter and a cool, low work function collector electrode. It is apparent from this description that decreasing the work function of the collector will increase the output power. There is a minimum acceptable collector Work function near 1 ev. which is set by the requirement that the collector must not itself emit considerable electrons. If high pressure plasma, such as cesium, is used, the exact relation of maximum output power to the work function difference between electrodes no longer obtains, but it is still true that lowering the collector Work function increases the output power.

The invention generally appertains to a thermionic converter having an improved low thermionic work function composition collector. In brief, the composite collector comprises a high work function metal substrate having a surface deposit or formation of an insulator coating or an insulator layer with varying thickness regions or spots. The outer surface of the insulator coating will be covered with atoms of a low ionization potential alkali metal, such as cesium, rubidium, potassium, sodium and lithium, present in appropriate regions of the converter to produce ions by contact ionization at the emitter to achieve space charge neutralization of thermionic electrons from the hot emitter.

The extremely reactive nature of alkali metals and their ability to chemically attack most substances necessitates that the insulator must be as inert to chemical attack by alkali metals as possible. Specifically, aluminum oxide (A1 0 is very inert to chemical attack of alkali metals, particularly cesium, and would suffice adequately for the purpose of the invention described herein. Other oxides, such as thorium oxide, lanthanum oxide, yttrium oxide and magnesimum oxide would also be resistant to attack by alkali metals, particularly cesium, on the basis of their large free energies, hence such oxides could be utilized. High work function substrate metals which could be used include aluminum, tungsten, tantalum, nickel, rhenium, iridium and molybdenum. There is no theoretical requirement that the insulator be the oxide of the corresponding substrate metal. If aluminum is used as the substrate metal or is deposited as a layer on top of another high work function metal, the aluminum or a portion of the aluminum can be oxidized by anodization or thermal oxidation to produce the aluminum oxide insulator layer.

The broad object of the present invention is to provide a thermionic converter having a very low thermionic work function collector.

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Another object is to provide a thermionic converter containing a vapor of a substance interposing the collector and emitter electrodes for neutralizing any potential barrier in the electron current path, wherein said substance is additionally used in conjunction with a composite material electrode to provide a low thermionic work function collector.

A more specific object is to provide a thermionic converter containing an alkali metal vapor interposing the emitter and collector electrodes, wherein the collector has a very low thermionic work function and is chemically stable in the presence of the alkali metal vapor.

A further specific object is to provide a thermionic converter containing cesium vapor interposing the emitter and composite collector eletcrodes, wherein the collector has a very low work function and is chemically stable in the presence of the cesium vapor.

Other objects, features and advantages of the invention will become readily apparent in the following description of the preferred embodiment of the invention when taken in conjunction with the appended claims and the attached drawings, wherein:

FIGURE 1 is an illustrative cross-sectional view of a conventional thermionic converter which may be modified by inclusion of a special, composite collector in accordance with the invention;

FIGURE 2 illustrates electron energy levels in a space chargefree converter operating under evacuated conditions (no alkali metal vapor) at maximum power output;

FIGURE 3 illustrates the electron energy levels in the converter of the invention without an alkali metal plasma utilizing a composite collector consisting of a metal substrate and an insulating layer;

FIGURE 4 illustrates the electron energy distribution with an alkali metal plasma present and alkali metal atoms condensed on the surface of the composite collector electrode;

FIGURE 5 depicts in a fragmentary perspective view the converter utilized in evaluating the invention; and

FIGURE 6 illustrates the improved contact potential difference achieved by the invention.

Referring specifically to FIGURE 1 there is shown 7 for illustrative purposes only a sectional view of a typical thermionic converter which is comprised of an electron emitter 2 situated and spaced in opposing relation to an electron collector 4, wherein the emitter and collector form part of an enclosure in conjunction with supporting members 6 and 8 and insulating spacer 10. The emitter electrode is conventionally a high temperature and high thermionic work function metal, for example tungsten, and the collector is any suitable metal inert to alkali metals at elevated temperature such as tungsten, molybdenum or nickel although the collector will 'have to withstand temperatures of only a few hundred degrees centigrade, perhaps 300400 C.

The emitter and collector of the converter depicted in FIGURE 1 are made in the shape of a circular disc, and the metal supporting members 6 and 8 for the emitter and collector, respectively, are made as very thin metallic diaphragms adapted to allow for thermal expansion and contraction welded to the peripheral portions of the electrodes. A tubular high temperature insulator, such as ceramic, is fixed to peripheral regions of the diaphragms as a supporting and spacing structure for the electrodes.

Heat, from any suitable source, is supplied to the emitter as shown in FIGURE 1, raising the emitter temperature sufficiently to cause substantial electron emission from the surface thereof. conventionally, thermionic converters are operated at emitter temperatures of about 1400 to 1900 C. and perhaps higher.

The walls of the metallic diaphragm 6 are made sufficiently thin so that very little heat conduction occurs between the emitter and the ceramic spacer 1t), and because of the poor thermal conductivity of the ceramic spacer very little heat is conducted from the metallic diaphragm 6 to diaphragm 8. The collector is maintained v at a much lower temperature, for example, several hundred degrees below that of the emitter.

An alkali metal reservoir 14 is provided adjoining opening 12 in the collector electrode for supplying alkali metal vapor in the space between electrodes. A small amount of alkali metal 16 contained in the reservoir is a suitable supply. When the atoms of alkali metal vapor strike the hot electron emitter, they are ionized and are emitted along with electrons into the region be tween electrodes as positive ions forming an alkili metal plasma therein. To prevent internal power losses due to scattering of electrons by the alkali metal vapor, the pressure of the metal vapor should be less than several torr depending on the distance between the emitter and collector. For example, if cesium is used, the pressure should be limited to about 2 torr by maintaining a reservoir temperature of about 300 C. The existence of the positive ions interposing the electrodes prevents the formation of any electron cloud or barrier to the electron flow from the emitter to the collector, all of which is well-known to those skilled in the art. By providing external electrical leads 18 and 20 in electrical connection with the. electron emitter and electron collector, respectively, a current can be supplied to load 21 to do useful Work.

Hereinbefore the physical description and operation of a thermionic converter utilizing an alkali metal plasma has been described. Now, with reference to the energy level diagram of FIGURE 2, a brief description of the thermionic converter operating with exact space charge neutralization at maximum power output will be presented. As illustrated, 5 represents the Fermi energy from the bottom of the conduction band to the Fermi level and represents the work function of the emitter; and and represent the same energies with respect to the collector. With the collector temperature at T and the emitter temperature at T I represents the electron current density from the emitter to the collector with the flat vacuum level indicating an exact space charge neutralization. As depiceted in the energy level diagram, FIG- URE 2, an electron current is produced by electrons escaping from the emitter and thus flowing to the collector. In order to be emitted, each electron must have a kinetic energy component internal of the emitter normal to the emitter surface which is equal to or greater than the emitter work function, and additionally the emitter Fermi energy, On arriving at the collector, each such electron must lose an amount of total energy 5 Electrons transferred from the emitter to provide electron current in the collector are raised in potential energy by an average amount for each elec* tron.

If an alkali metal plasma is added in excess of that re quired for space charge neutralization, the potential energy between the electrodes will be lowered. The thermionic Work function of the bare metal collector will be lowered through adsorption of the alkali metal on the surface thereof. By reduction of the thermionic work function of the collector, the kinetic energy consumed by the electron. after passing through the lesser collector surface potential barrier will be decreased and thus provide i an attendant increase in the amount of available energy or voltage for doing external work.

It is well-known (A. J. Decker, Solid State Physics, Prentice-Hall, Inc., 1959, pp. 228-230), that positive ions adsorbed onthe surface of a metal create a dipole layer which produces a voltage drop AV that effectively reduces the thermionic work function of the metal. That is, if positive ions are adsorbed on the surface of the metal, there are equal and opposite image charges formed in the metal which, together with the positive ions, cause a potential drop between the metal surface to the center of positive ions. It is therefore easier for an'electron to escape or to enter the metal as a result of the electric field within the dipole layer. Normally, with the ions residing directly on the bare metal surface, the distance between the positive and negative charges forming the dipole layer is in the order of the ion diameter, say about 10* centimeters, Thermionic converters operating with an alkali metal vapor afford a collector exhibiting the same thin layer dipole.

In such converters the amount of the voltage drop, AV, between the positive ions and the metal surface collector is proportional to the distance separating them, hence the thermionic work function is reduced by an amount equal to the voltage drop, AV. By increasing the space between the positive and negative charges forming the dipole layer, the voltage drop can be proportionally increased, further reducing the thermionic work function. However, in practice for a conventional thermionic converter, there is no way to separate the positive ions and the image charges forming the dipole layer by vacuum spacing greater than the normal spacing formed by the positive ions adsorbed on the metal surface.

According to the present invention, an appropriate insulator is provided on the surface of the substrate metal with positive ions of an alkali metal adsorbed on the surface of the insulator, thus forming a dipole layer with an increased spacing between the positive ions and the image charges, the spacing being larger than can be obtained with positive ions adsorbed on a metal substrate surface.

Since an insulator must be used to increase the dipole spacing, it is also readily apparent that the dielectric constant of the insulator reduces the voltage drop due to the dipole layer thereof. Thus, the system is equivalent to a capacitor in that the voltage drop due to the dipole layer through the oxide is also inversely proportional to the dielectric constant of the aluminum oxide or other insulator that may be used.

FIGURE 3 illustrates the various energy levels of the thermionic converter depicted in FIGURE 1 modified according to the invention to include an insulating layer collector but without an alkali metal vapor present, yet operating under conditions of exact space charge neutralization and maximum power output. Distinctions between the energy level diagrams of the insulating layer collector and metal collector will be observed by comparison of FIGURES 2 and 3.

The insulating layer under consideration has a thickness much smaller than the Debye shielding length for the insulator, the Debye shielding length being defined as that distance in which a system of free charge carriers will reduce an electric field or an electric field from another charge to 1/ e of its original value by means of rearrangement of carriers. If such insulator exhibits an electron affinity of the potential barrier at the metal insulator interface of the composite collector is reduced from the metal work function gb to an amount as illustrated in FIGURE 3. However, the thermionic work function of the composite collector is still the thermionic work function of the substrate metal. For an insulator thickness exceeding the Debye length, a potential gradient would occur in the insulator which, with an insulator several Debye lengths thick, permits the thermionic work function of the composite collector to approach the work function of the bulk insulator.

The operation of the thermionic converter having an insulating layer of appropriate thickness is such that when an electron in the emitter has a kinetic energy equivalent to normal to the emitter surface, it will overcome the surface potential barrier of the emitter and be transferred to the collector where it will pass through two distinct potential barrier drops, namely Xox and E or Thus, it will be recognized that the thermionic work function of the composite collector is the same as that of the substrate metal.

. In order to lower the thermionic work function of the composite collector, an alkali metal must be adsorbed on the outer surface of the insulator. When the alkali atom is adsorbed, any one of several mechanisms can cause a voltage drop through the oxide which produces the decrease in work function. Two mechanisms using ions of alkali metal that produce the necessary potential drop, AV, to reduce the work function of the composite collector will be presented.

In the first instance, a thin insulator which is comparable to several electron wavelengths (or about 50 angstroms) permits electrons to pass easily from the alkali metal atom adsorbed on the outer surface of the insulator, through the insulator, and thus into the substrate metal. Thereby, the collector substrate metal causes ionization of the alkali metal. This contact ionization requires that the work function of the substrate metal be higher than the ionization potential of the alkali atom.

In another mechanism which occurs with very thick insulators, in comparison to several electron wavelengths, the potential drop or dipole is established without the reliance on electron transfer from a surface adsorbed atom to the substrate metal. Such a mechanism is dependent on exposure of the insulator to an alkali metal plasma (as in the thermionic converter disclosed herein). It will be recognized if an electron from the alkali metal atom on the insulation surface cannot easily traverse the insulator to establish a dipole then alkali metal ions may be adsorbed on the insulator surface from the alkali metal plasma and cannot be readily neutralized. Hence, the ions will establish a potential drop which will lower the work function of the composite collector.

FIGURE 4 shows the energy level diagram when alkali ions are adsorbed on the outer surf-ace of the insulator and any of the mechanisms above are active. The voltage drop through the insulator is given by the dipole moment of the ions and their images in the substrate metal as modified by the dielectric constant of the insulator as previously discussed. In both cases, the alkali atoms or ions on the insulator surface will be polarized attributable to the proximity of the outer surface of the insulator resulting in a reduction of the electron affinity of the insulator from Xox to If the work function of the substrate metal is larger than the ionization potential of the alkali atom, at least one of the above mechanisms would be active. If the reverse is true, the first mechanism would not be active.

The actual value of the thermionic work function, although created by the potential drop in the insulator, is dependent on the insulator thickness. For thicknessess less than a Debye length, the thermionic work function of the composite collector can be derived from one of two models depending on the insulator thickness. The first model is one where the insulator thickness is less or comparable to several electron mean free paths for energy loss in the particular insulator, and the second model is one where the insulator thickness is much larger than the mean free path for energy loss.

In the first case electrons will pass through the insulator with little or no energy loss so that the thermionic work function of the composite collector is determined by the highest potential barrier whether it be at the outer surface of the insulator or at the metal insulator interface. Using any of the mechanisms which could cause the potential drop through the insulator, it is quite probable that the voltage drop would be larger than the electron alfinity, If this is the case, the work function will be determined by the potential barrier at the metal insulator interface. Thus, the thermionic work function will basically be If the insulator is very thin, the thermionic work function will also be decreased by means of a Schottky effect, A as depicted in FIGURE 4, and for extremely thin insulators an even greater decrease will be experienced due to tunnelling of electrons through the potential barrier.

In the case of thicker insulators, electrons will lose all their excess kinetic energy in the oxide before arriving at the metal insulator interface. Thus, the thermionic work function will be determined entirely by the barrier at the outer surface of the insulator. This barrier will also be quite low due to the potential drop through the insulator. However, the internal resistance of the insulator layer provides considerable internal power loss in the device when large currents are emitted from the emitter. This power loss factor will ultimately limit the thickness of the insulator. The current in the insulator may be larger than would be predicted on the basis of the bulk resistivity because of current carriers injected into the insulator from one of the metal electrodes. This injected current will be limited by its own space charge in a manner very similar to space charge limitations in a vacuum. The voltage drop across the insulator to produce the necessary current flow will cause an internal power loss.

Two conditions must be met in order to observe appreciable space charge current in the insulator: (l) at least one ohmic contact, and (2) a relatively small number of trapping defects. The metal insulator interface should be ohmic for electrons because the metal work function is less than that of the insulator. Furthermore, large current can be injected into the composite collector because the electrons come from a hot emitter spatially separated from the cooler collector. Shallow traps can be taken into account by reducing the mobility, u, to 0,41. when 0 is the fraction of total charge injected which is free. In this case the current in the presence of space charge can be written as 2 I=10" amps/cm. where V is the voltage drop across thin film of thickness d. The free carrier mobility is ,u. and K is the dielectric constant, both of the insulator.

In a practical thermionic diode, the output voltage may be the order of 3 volts and the output current, at best, about 5 amps/cmF. For the internal power loss to be less than 10 percent, the voltage drop across the insulator must be less than 0.3 volt. The output current must also pass through the insulator. Thus, for 10 percent internal power loss when operating at 3 volts and 5 amps output power The mobility ,u. is directly proportional to the electron mean free path previously mentioned. The constant of proportionality will depend on the scattering mechanism.

Since the unknown factor, 9/5 K w, occurs in the onethird power, my estimate of its value can have confidence limits of three orders of magnitude with the result of confidence limits of one order of magnitude on d. The

dielectric constant will not vary much from insulator to The thickness d will be the order of 1000 A. if ,u6 is between about 10- and 10. For A1 0 0 would have to be between 10" and l. The lower part of this range is about as large as can be expected for A1 Certainly any of the insulators under consideration will have a 0 which cannot be larger than the range of 0 quoted above. Therefore, the estimated maximum d will be in the order of magnitude of 1000 A. for all the insulators under consideration.

In summary the requirement for low thermionic work function with such a composite collector is for an alkali vapor to be present around the collector. The lowest thermionic work functions are to be expected for the thinnest insulators. Practical application of these electrodes as thermionic collectors will limit the thickness of the insulator to several thousand angstroms because of the need to minimize power dissipation caused by ohmic losses in the insulator.

Throughout the foregoing, the invention has considered the presence of ions formed by some mechanism being active. Ion activity, as such, appertains to the preferred embodiment of the invention; however, the presence of ions on the surface of the insulator is not absolutely essential. In fact, for either very thin or very thick insulators it is unnecessary for ions to be adsorbed on the surface. The insulator surface need only adsorb neutral alkali atoms to produce the necessary potential drop, AV, to achieve a decreased work function composite collector. When one or more monolayers of neutral alkali atoms are adsorbed on the insulator surface, the properties thereof more closely approach the characteristics of the bulk alkali element rather than alkali metal vapor atoms. Hence, the potential in the insulator is describable as a metal-insulatormetal element with an energy level diagram appearing much the same as that for surface adsorbed ions. The potential drop, AV, will be approximately equal to the difference in work functions between the substrate metal and the alkali metal.

Referring to FIGURE 5, there is illustrated a thermionic converter embodying the features of the invention which has been operated under varying conditions to achieve improved power conversions. As illustrated, the converter consists of a wire emitter 50 made of tungsten having a mil diameter, and a composite collector cylinder 51 having an insulator coating 52 thereon. The internal diameter of the collector cylinder 51 is about 1.4 centimeters and the width of cylinder 51 is .6 centimeter. Cylinder 5-1 is aluminum which has been anodized to form the aluminum oxide (A1 0 insulator layer 52. A pair of grids 53 and 54 having internal diameters of 1 centimeter arranged in end-opposed relationship form an 0.02 centimeter wide slit circumferentially between the grids. Each grid cylinder is 2.8 centimeters long. The grids are made of tantalum. Although not illustrated, the entire assembly is mounted in an evacuated glass housing with a source of cesium vapor therein.

Although the preferred physical embodiment is illustrated in FIGURE 1, the preferred materials of construction and alkali-metal plasma of the invention are set forth hereinafter. Several diodes were made in accordance with the physical description illustrated in FIGURE 5 which embodied various features of the invention. The diodes used aluminum collectors whose surfaces were oxidized to form a thin protective and insulating layer. The aluminum oxide film was formed in one case by controlled thermal oxidation of the aluminum substrate and in other cases by anodization of the aluminum in an ammonium pentaborate solution of ethylene glycol. Other anodizing solutions could be used. Various thicknesses of the insulatingcoating of aluminum oxide have been used; for example, 30 A., 300 A. and 2000 A. thick coatings have been utilized in the invention. The thermionic converters were operated using cesium at reservoir temperatures of 30 to 67 C., which controlled the vapor pressure of cesium atoms within the converter.

Table I hereinafter presented contains specific examples of the thermionic converter of the invention operated at a tungsten-emitter temperature sufiicient to produce a true emitter work function of 4.6 ev. at various collector temperatures and cesium reservoir temperatures. The composite collector is compared with a tungsten collector, all operating with adsorbed cesium on the collector surface.

TABLE I Collector Material Ta i, T05, CPD, 1 001 Example No. (Coated With C. C. volts ev.

Cesium) 100 67 2. 8 1. 8 1 Tungsten 450 67 2. 9 1. 7 2 30 A. coating of A1203 011 Al. 70 30 3. 2 1. 4 d0 100 30 3. 2 1. 4 10.. 84 67 3. 2 1.4 112 67 3. 2 1. 4 0 A. coat g of Alg03 on Al. 70 30 3.1 1. 5 7 do 100 30 3.1 1. 5 84 67 3. l 1. 5 112 67 3. 1 l. 5

FIGURE 6 graphically illustrates the improved output voltage and electron current achieved in accordance with the invention. In FIGURE 6 the electron current versus output voltage is plotted on a logarithmic scale in which curve A, curve B and curve C illustrate the operation of the thermionic converter with a 30 A. aluminum oxide film, a 300 A. aluminum oxide film and a tungsten collector surface, respectively, all coated with cesium, Points A, B and C represent the projected contact potential difference CPD. It will be noted that curve A shows an increase voltage output over curve B, and both curves A and B show an increase output voltage over the normal tungsten collector, curve C. J represents the saturated current depicted in FIGURE 6.

In the foregoing analysis, the work function of the collectors was obtained from the contact potential difference, V in terms of ev. between emitter and collector work functions which were obtained from the intersection of the saturated electron emission current, J and the current for decelerating voltage as depicted by a logarithmic graph of current versus voltage. All voltages, in volts, were referred to the grounded collector and were equal in magnitude to the potential energy in electron volts. Thus, the contact potential difference in volts, CPD, was related to the contact potential difference and the work functions expressed in ev. according to the following equation:

CPD (volt) =V (ev.) (ev.) (ev.)

where A is amps/cm. K o is the emitter work function, k is Boltzmanns constant, and T is the emitter temperature. The saturated emission current will be obtained when the applied voltage accelerates electrons from the emitter to the collector. This occurs when the voltage on the emitter is positive and less than CPD or negative. In actual practice, patchiness or non-uniformness of the emitter surface deforms the current-voltage curve near the CPD.

In the region of decelerating voltages, which occurs when the emitter voltage is positive and greater than the CPD, the logarithm of the current can be easily interpreted when both emitter and collector are plane sheets which face each other. In plane geometry the current is for V V.,:J=J exp where V is the potential in electron volts on the emitter for grounded collector. When V=V this last equation reduces to the saturated current. In actual practice, patchiness will again distort the current near V It can be easily Thus, the current for V V will be a straight line when plotted as log l versus voltage with its slope inversely proportional to the emitter temperature. The emitter temperature thus serves as a check on the validity of the current-voltage data.

Since plane geometries are difficult to fabricate for evaluation, in actual practice it is much easier to test using other geometries, applying well-known geometrical corrections. In the thermionic converter of FIGURE 5, two corrections had to be applied: (1) a cylindrical correction to transform the data from cylindrical to plane geometry and (2) a correction. to account for the defocusing of the electron beam by the grid.

In cases where only a wire emitter and a plane collector were used, the geometrical correction can only be estimated. In such a case, however, a comparison of two collectors in an identical position will yield the difference between their work functions.

In Examples 2-5 the converter depicted in FIGURE 5 was used, whereas in Examples 6-9 the data was taken in a thermionic converter having a pair of plane geometry collectors (one having a 30 A. A1 0 coating and the other a 300 A. A1 0 coating) with a common wire emitter. Example 1 is taken from available literautre and experiments in the laboratories of Texas Instruments Incorporated.

These devices were operated at relatively high emitter temperatures where the emitter is free of cesium and its work function is very near 4.6 ev. Others were operated using an emitter temperature below 1000 K. such that the emitter work function was 1.7 ev. Actually, the emitter might be operated at an intermediate temperature such that its work function is near 2.5 ev. In such cases a secondary source of ions might be needed since ions are usually produced by constant ionization of the alkalimetal plasma at the surface of the high work function emitter.

In view of the foregoing disclosure, it will become readily apparent to those skilled in the art that various modifications and changes with regard to the structural materials, design and geometries may be made, and such modifications and changes are within thescope and intent of the invention as defined by the appended claims.

What is claimed is: 1. A collector electrode for a thermionic energy con- 10 verter comprising a metal substrate, an intimate insulating layer on the outer surface of said substrate, said layer having a thickness less than a Debye length for said material and having in intimate contact with the surface of said insulating layer at least a monolayer of an alkali metal selected from the group consisting of cesium, rubidium, potassium, sodium and lithium.

2.. The collector electrode of claim 1, wherein said insulating layer is aluminum oxide having a thickness no greater than about 300 A. and the alkali metal is cesium.

3. The collector electrode of claim 1, wherein said insulating layer is aluminum oxide having a thickness no greater than about 30 A. and the alkali metal is cesium.

4. In a thermionic device, a low work function electrode comprising a metal substrate, an intimate insulating layer on the outer surface of said substrate chemically resistant to alkali metals, said layer having a thickness less than a Debye length and being at least partially coated with alkali metal atoms.

5. In a thermionic device, a low work function electrode comprising a metal substrate, an intimate insulating layer on said substrate chemically resistant to alkali metals, said layer having a thickness no greater than the order of magnitude of 1000 A. and being at least partially coated with alkali metal atoms.

6. The device of claim 5, wherein said metal atoms are selected from the group consisting of cesium, rubidium, potassium, sodium and lithium.

7. In a thermionic energy converter, the improvement comprising a collector electrode formed of an aluminum substrate having an aluminum oxide layer at its outer surface, said layer having a thickness less than the debye length for aluminum oxide and coated with at least a monolayer of cesium atoms.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner.

ORIS L. RADER, MILTON O. HIRSCHFIELD,

Examiners.

WILLIAM L. JARVIS, I. W. GIBBS,

Assistant Examiners. 

1. A COLLECTOR ELECTRODE FOR A THERMIONIC ENERGY CONVERTER COMPRISING A METAL SUBSTRATE, ANINTIMATE INSULATING LAYER ON THE OUTER SURFACE OF SAID SUBSTRATE, SAID LAYER HAVING A THICKNESS LESS THAN A DEBYE LENGTH FOR SAID MATERIAL AND HAVING IN INTIMATE CONTACE WITH THE SURFACE OF SAID INSULATING LAYER AT LEAST A MONOLAYER OF AN ALKALI METAL SELECTED FROM THE GROUP CONSISTING OF CESIUM, RUBIDIUM POTASSIUM, SODIUM AND LITHIUM. 